(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to fabricate an image sensor device.
(2) Description of the Prior Art
Active pixel sensor, (APS), cells, are usually comprised with a photodiode element, formed in a semiconductor substrate, as well as transfer transistor, and reset transistor structures. The peripheral regions of the APS cell contain the needed control and processing circuits, usually comprised of complimentary metal oxide semiconductor, (CMOS), devices. Therefore process sequences are chosen so that sharing, or simultaneous processing, for all needed devices, such as the photodiode element, and the transfer and reset transistor, as well as the peripheral devices, can be employed.
The photodiode element of the APS cell, which collects light, which is later converted to electrical signal, is sensitive to specific process steps or sequences. Damage to the surface of the photodiode element, as a result of exposure to specific process steps, such as reactive ion etching, (RIE), can result in unwanted dark current generation, for the APS cell. One such exposure of the photodiode to RIE procedures, occurs at the conclusion of a contact hole opening, formed in an insulator layer, exposing a portion of the top surface of the photodiode region for subsequent contact from an overlying metal structure. To insure opening of this contact, a RIE overetch cycle is sometimes used, resulting in the unwanted damage of the surface of the photodiode element, and the resulting deleterious dark current generation. This invention will offer methods that reduce, and prevent this type of RIE damage of the photodiode element, via the use of a polysilicon pad structure, placed overlying the photodiode element, thus absorbing the consequences of the RIE, contact opening procedure. Prior art, such as Chi, in U.S. Pat. No. 5,854,100, show an APS cell, however with the contact opening made directly on the top surface of the photodiode element.
It is an object of this invention to fabricate an active pixel sensor, (APS), cell, using a polysilicon pad structure located on a portion of the top surface of the photodiode element, to protect the photodiode element from RIE damage, occurring during the procedure used to open a contact hole to expose the photodiode element.
It is another object of this invention to form the polysilicon pad structure, on a thin oxide layer overlying the photodiode element, simultaneously with the formation of a polysilicon gate structure of a reset transistor structure, using a small diameter opening in the thin silicon oxide layer, created using RIE procedures, for contact of the polysilicon pad to the photodiode element.
It is still another object of this invention to form the small diameter opening, in a thin oxide layer, located on the photodiode element, using a wet etch procedure.
In accordance with the present invention a method of fabricating a APS cell, featuring the use of a polysilicon pad structure, located between an underlying photodiode element, and an overlying metal contact structure, is described. After formation of a photodiode element, in a semiconductor substrate, comprised of an N+ region, in a P well region, a gate insulator layer, and a thin polysilicon layer, are formed. A small diameter opening is created in the thin polysilicon layer, and in the gate insulator layer, exposing a portion of the top surface of the photodiode element, using either a RIE procedure, or an RIE procedure for the thin polysilicon layer followed by a wet etch procedure for the gate insulator layer. After deposition of a thick polysilicon layer, conventional photolithographic and RIE procedures, are used to define a polysilicon pad, comprised of the thick polysilicon layer, and the thin polysilicon layer, on the gate insulator layer, contacting the photodiode element, in the small diameter opening. The same procedure creates a polysilicon gate structure, for an adjacent reset transistor structure, comprised of the thick polysilicon layer, overlying the thin polysilicon layer, on the underlying gate insulator layer. After formation of insulator spacers, on the sides of the polysilicon gate structure, a source/drain region is formed in an area of the semiconductor substrate, used for the reset transistor, not covered by the polysilicon gate structure. A contact hole opening, is then formed in an interlevel dielectric, (ILD), layer, exposing a portion of the top surface of the polysilicon pad structure, followed by the formation of the metal contact structure, in the contact hole opening, communicating with the photodiode element via the polysilicon pad structure.